The Original Bird Flu: Chlamydia psittaci

By Abigail Korenek

Peer Reviewed

Almost 100 years ago, a different kind of bird flu captured the public consciousness. Today, the media cycle is dominated by reports of highly pathogenic avian influenza in commercial agriculture operations. Over 148 million birds have been ordered to be culled since H5N1 first reached American commercial farms in 2022, resulting in skyrocketing egg prices and empty shelves at the grocery store.¹ Even more alarming are the 70 documented human cases in the US, which indicate that the virus can successfully “spill over” into humans, replicate, and potentially mutate.² However, in late 1929, the threat had a seemingly more innocuous source. Parrots were the Christmas gift of choice that year, and little attention was paid to the occasional sickly bird; that is, until the owners began to fall ill as well.³ The illness was no sniffle, either: while some parrot owners recovered quickly from a flu-like illness, others progressed to fulminant pneumonia, resulting in hospitalization and sometimes death. Reports of a mysterious and exotic “parrot fever” took the media by storm, leading to a national frenzy that saw pet birds abandoned in the streets en masse.⁴

The mysterious parrot fever was no mystery after all: psittacosis was first described in the late 19th century by Swiss physician Dr. Jakob Ritter. At the time, the causative agent remained unknown (and would continue to do so until the 1960s); however, Dr. Ritter had astutely connected the cage of parakeets in the study with an outbreak of atypical pneumonia in his family home that killed Dr. Ritter’s brother and two others in 1880.^5,6 Despite his loss, Dr. Ritter meticulously documented the outbreak and published his findings in “Contribution to the Question of Pneumotyphus,” an elegant epidemiological analysis of his tragedy. This would not be the first personal sacrifice for science in the history of psittacosis. In his article, Dr. Ritter outlined the incubation period and natural history of the disease and went so far as to hypothesize that the disease was infectious, but not contagious (he was almost correct). However, this article was published before germ theory had been fully worked out, and Dr. Ritter did not identify an etiology.⁵

Psittacosis, also called ornithosis, is now known to be caused by Chlamydia psittaci, an intracellular bacterium with a unique biphasic lifestyle. The bacterium alternates between two forms, a metabolically inert but infectious form called the elementary body (EB) and the replicative form, called the reticulate body (RB).⁷ Notably, the EBs are hardy and remain infectious in the environment for weeks to months.⁸ The EBs are primarily spread via inhalation of dust contaminated with droppings, feathers, and secretions from infected birds, but rare transmission events between humans have been reported.⁷ In birds, C. psittaci infection manifests as lethargy, weight loss, respiratory distress, reduced egg production, and significant mortality.⁹ Infection in humans is characterized by the abrupt onset of fever, myalgias, nausea, vomiting, diarrhea, and cough. Untreated, the organism disseminates hematogenously, resulting in an array of multisystem manifestations that can be fatal. Fortunately, psittacosis is now highly treatable with doxycycline, when recognized early.^10,11

Unfortunately, in the early 1930s, doxycycline would not be invented for another three decades.¹² In an eerily familiar turn of events to COVID-19 and avian influenza alike, the panic surrounding the outbreak turned into ridicule and political fodder almost as quickly as it began. Suddenly, parrot fever became a national laughingstock and was referred to as “the latest and most amusing example of the national hypochondria,” no doubt related to its propensity to infect housewives and widows in need of a pet.^4,13 However, a group of determined scientists at the Hygienic Laboratory in Washington were convinced that psittacosis was no laughing matter. They dutifully carried out their experiments on sick birds in search of the elusive causal agent, and a possible cure. Until they, too, started to fall ill. Alarmingly, several succumbed to their illness in the line of scientific duty, harkening back to the fate that befell Dr. Ritter’s family.¹⁴ Included among those who died of psittacosis were Dr. Daniel S. Hatfield, Hatfield’s colleague Dr. William Stokes, and “Shorty” Anderson, a laboratory technician. All had been infected by their experimental parrots.⁴

Dr. Charles Armstrong, who had been leading the scientific investigation, fell ill with parrot fever soon after. In an act of desperation, Dr. George McCoy, the director of the Hygienic Laboratory, drew blood from a convalescent patient and injected it into Dr. Armstrong, who ultimately survived. The Hygienic Laboratory was deemed contaminated, and Dr. McCoy single-handedly performed the grisly task of culling every research animal before the fumigation team arrived to decontaminate the premises with cyanide.⁴ The national conversation regarding psittacosis quickly sobered in light of the tragedy. All told, 169 cases with 33 deaths were recorded in the US. Sixteen of these cases occurred in connection with the Hygienic Laboratory. Similar small outbreaks unfolded in 20 additional countries, totaling approximately 800 cases with an overall mortality rate of approximately 15% worldwide–a proper pandemic.¹³

Today, psittacosis remains an underrecognized and underdiagnosed disease: less than ten cases are typically reported per year in the US, despite a high global burden of chlamydial infections in avian reservoirs.^7,15 Nevertheless, C. psittaci has been rightfully designated as a CDC Category B biological agent to reflect its propensity to disseminate and cause deaths (other Category B agents include ricin toxin and certain foodborne infections).^7,16–18 Fortunately, while psittacosis remains underdiagnosed, the important contributions and tireless work of the scientists at the Hygienic Laboratory did not go unrecognized. Their tragedy brought national attention to the necessity of federally funded scientific research, and for their efforts and sacrifice, they were awarded $750,000 (worth approximately $14,000,000 today) for two new buildings. They also earned a new name: the National Institutes of Health.³ The move from Washington to Bethesda, Maryland, followed shortly thereafter.¹⁹

At the time of the psittacosis pandemic, there were 121 employees at the Hygienic Laboratory.¹⁴ Today, the NIH boasts over 20,000 employees across 27 different institutes and centers, not to mention the countless scientists and discoveries supported by their grants across the country.²⁰ NIH researchers discovered how the genetic code is translated into proteins, discovered that fluoride prevents tooth decay, and contributed to multiple vaccines, among dozens of other foundational discoveries.²¹ To date, 174 Nobel laureates have been supported by NIH funding.²² The psittacosis pandemic of 1929-30 is a largely unknown success story that represents the enormous value of funding science during times of crisis and the great sacrifice made by scientists to protect the health of the American public. The impact of the NIH on global medical science and human progress cannot be overstated, and yet, we find ourselves at a crossroads once again. Infectious threats loom large but have instead been met with active divestment from the NIH. One can only hope that it will not require another tragedy to inspire investment in the medical sciences once again.

Abigail Korenek is a Class of 2027 medical student at NYU Grossman School of Medicine

Reviewed by Michael Tanner, MD, Executive Editor, Clinical Correlations

Image courtesy of Christian Matos, CC0, via Wikimedia Commons

References

The Original Bird Flu: Chlamydia psittaci

October 28, 2025

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